What is EMI or RF

[zener]

Just a newbie wanting to learn more  :

What is this EMI or RF that should be prevented from getting into your circuit? Where it comes from and what it does when it gets to a circuit?

Thanks for any answer  

Ner

[C Bradley]

EMI is electro-magnetic interference.
RF is radio frequency interference.
both are causes of noise.

Chris B

[Peter Snowberg]

RF or RFI is Radio Frequency Interference, which is a sub-category of Electro-Magnetic Interference. EMI covers the whole spectrum from far below radio frequencies to super high frequencies.

EMI is commonly produced by devices that switch power (switches, relays, motors [the brushes are constantly switching]), and devices that operate from alternating current plasma (such as fluorescent and neon lights).

Shielding is the way to keep it out.

Take care,
-Peter

[zener]

I was suggested by a friend to use shielded wire instead of the usual stranded. There's a plastic strand aside from the copper strands inside the shielded wire. What am I going to do with it?

[Peter Snowberg]

I don't quite understand.... Shielded wire has the shield layer which is usually braided, then another layer of plastic insulation, and then the wire your signal travels through at the center. You only want to ground the shield wire at one end most of the time. If you ground it at both ends, it will create a ground loop, which can act like a transformer and add hum to your audio.

Sometimes the wire in the center is quite small. Carefully cut into that plastic in the center and you should find the wire to run your signal on.

Take care,
-Peter

[zener]

Sometimes the wire in the center is quite small. Carefully cut into that plastic in the center and you should find the wire to run your signal on.

Take care,
-Peter

That might be it. The wire I have is a black one. If you try to peel off the black rubber skin what you'll find is some strands of copper and small white plastic/rubber strand. I still haven't figure out if there's a tiny strand of wire inside that white plastic/rubber strand.

Thanks.

Zener

[Peter Snowberg]

Sometimes the center conductor can get pulled back into the cable a little bit. The plastic and the braided layer stretch more than the center conductor. Try clipping off a centimeter of that inner plastic insulation and you may suddenly see the signal conductor.

Take care,
-Peter

[MAXIMUS]

is the outside layer of braided wire for interference sheilding?
...curious

[Peter Snowberg]

is the outside layer of braided wire for interference sheilding?
...curious

Exactly.

Take care,
-Peter

[zener]

Sorry Peter, but I can't still figure it out. There's a tiny wire inside the white rubber strand. Outside the plastic strand, there are some copper strands. If I'm going to use the wire inside the white rubber strand, what am I going to do with those copper strands outside?

You mentioned something braided. I don't know what it is or what I have is not really a shielded wire.

Thanks.

Zener

[Peter Snowberg]

It sounds like you have coax cable there.

For most effects box use, the center conductor will be used just like regular wire, and the shield will be grounded at one end but not both. The other end of the shield is just cut off so that it doesn't connect to anything.

This is a description of the layers:
Outside jacket (usually PVC plastic)
Shield (usually a braid of copper strands)
Inner Insulation (sometimes made of foamed plastic, sometimes rubber, sometimes nylon)
Center Conductor (either stranded or a single conductor)

If you cut the outside jacket off the cable for a few centimeters, most coaxial cable will display a braided shield. Not all cable is like this. Some cable uses a conductive plastic shield and the copper in the shield is just wrapped around the conductive plastic. Some coax uses a foil shield.

If you don't have another ground between two points (like between the input jack and the circuit board, then you can use the shield just like a ground wire. If you already have a ground wire, then just connect the shield at one end.

I hope that helps,
-Peter

[zener]

I hope I get it this time (or I'll call myself dumb once again). :roll:
In the input jack, one wire is the input wire and the other is to the ground. I'll use the wire inside the white rubber for the input and the braided copper surrounding the white rubber to the ground.

So, a shielded wire can have two wires (for signal and for ground) in one .

Thanks Peter for bearing with me :wink: .

Zener

[Peter Snowberg]

It sounds like you have it.

Dumb? No possibility of it. We all go through learning and then hopefully we all pass along what we have learned.

Take care,
-Peter

[Paul Marossy]

"What is EMI or RF?"

A pain in the butt! Fortunately, I've never had a problem with that so far, but there's a first time for everything. Usually, just using a metal enclosure works just fine. I've never even had to use a shielded input wire... some people experience problems with high gain circuits, and it becomes necessary to use it.

Most of my problems are hum from my guitar pickups due to EMI, not anything in the effects themselves.